REACTIONS IN MICROEMULSION MEDIA. BOROHYDRIDE REDUCTION OF MONO- AND DICARBONYL COMPOUNDS

Article abstract of DOI:10.1016/S0040-4020(01)96887-5

Microemulsions were prepared at 26 deg C from mixtures of hexanes (O), a 50:1 (w/w) solution (W) of 0.1 M KOH-NaBH4, and a 1.23:1 (w/w) mixture (S) of hexadecyltrimethylammonium bromide (HTABr) and 1-butanol.A pseudoternary phase map contained a significant microemulsion (μE) region, and μE's A and B (60:35:5 and 20:10:70 S:W:O, respectively) were used for reduction of several monocarbonyl compounds , an α,β-unsaturated ketone , and a diketone <4-(4'-benzoylphenyl)-2-butanone (7a)> at 26 deg C.For comparison purposes, reductions were also performed in aqueous 2-propanol (2-PrOH A and 2-PrOH B) prepared by the substitution of 2-propanol for the S and O components of μE's A and B.Generally, the reductions were slightly faster in the microemulsion media than in the corresponding aqueous 2-propanol media.The significantly slower reduction of 4a relative to that of 3a in μE B indicated that the interphase is the reactive site.With enone 6a, the influence of microemulsions on the competition between 1,2- and 1,4-reduction was determined.In μE's A and B there was 8percent and 11percent 1,4-reduction, respectively, whereas in 2-PrOH A and B there was only a trace.With diketone 7a, the reactivity of the aromatic carbonyl group relative to that of the aliphatic carbonyl group increased on going from 2-PrOH A and B to μE's A and B, respectively.For the sodium borohydride reduction of ketones, microemulsion catalysis is more effective than phase transfer catalysis or the use of a tetraalkylammonium borohydride in a hydrocarbon solvent.